The present invention relates generally to an amplifier suitable for driving the servomotor of a video tape recorder or the like and, more particularly, to an amplifier which quickly amplifies signals by a specific amplification factor based on a reference voltage.
In FIG. 1 there is shown a conventional amplifier.
A signal e.sub.i is passed to an inverted input of an operational amplifier 6 via a first capacitor 2 and a first resistor 4. An output signal from the operational amplifier 6 is fed back in to the inverted input via a second resistor 8. The resulting amplification factor A of the operational amplifier 6 is given by: EQU A=R.sub.2 /R.sub.1
where R.sub.1 and R.sub.2 are the resistance values of the first resistor 4 and of the second resistor 8, respectively.
A supply DC voltage V.sub.b, which is supplied by a power source, is divided by a voltage divider containing a third resistor 10 and a fourth resistor 12. The voltage is passed to a noninverted input of the operational amplifier 6. The divided voltage is given by: EQU {R.sub.4 /(R.sub.3 +R.sub.4)}.V.sub.B
wherein R.sub.3 and R.sub.4 are the resistance values of the third resistor 10 and the fourth resistor 12, respectively. Although the divided voltage is constant under a fixed supply voltage V.sub.B, the supply voltage V.sub.B may change due to noise contamination, thereby varying the divided voltage. In order to overcome this drawback, a second capacitor 14 is connected in parallel with the fourth resistor 12. Once charged, the second capacitor 14 allows a reference voltage V.sub.REF (V.sub.REF ={R.sub.4 /(R.sub.3 +R.sub.4)}. V.sub.B to remain constant, regardless of variation in the supply voltage V.sub.B. In the present invention, an output e from the operational amplifier 6 is given by (V.sub.REF +A. e.sub.i), provided that the second capacitor 14 is charged to the reference voltage V.sub.REF and that the inverted input voltage of the operational amplifier 6 is equal to the reference voltage V.sub.REF.
When the supply voltage V.sub.B is provided by a power source, the second capacitor 14 is charged via the third resistor 10 up to the value of reference voltage V.sub.REF (see FIG. 2(1)). The first capacitor 2, however, is charged by the output of the operational amplifier 6 via the second resistor 8 and the first resistor 4, at a slower rate than second capacitor 14. Thus, a considerable time lag exists between the time the inverted input voltage of the operational amplifier 6 equals reference voltage V.sub.REF and the second capacitor 14 fully charges (see FIG. 2(2)). In order to solve this problem, the conventional amplifier incorporates a diode 102 in the operational amplifier 6 in the forward direction between the noninverted input and the inverted input.
In an amplifier which incorporates diode 102, the first capacitor 2 is initially charged by the output of the operational amplifier 6 via the second resistor 8 and the first resistor 4 (see a in FIG. 2(3)). When the noninverted input voltage of the operational amplifier 6 exceeds the inverted input voltage by a forward voltage V.sub.F of the diode 102, the diode 102 conducts. This causes the first capacitor 2 to rapidly charge via the third resistor 10, the diode 102 and the first resistor 4, which in turn rapidly increases the inverted input voltage of the operational amplifier 6 once the second capacitor 14 has charged (see B in FIG. 2(3)). Further, the second capacitor 14 is charged to obtain the reference voltage V.sub.REF, leading the noninverted input voltage of the operational amplifier 6 to the (VREF-V.sub.F) level. This causes the diode 102 to open. As a result, the first capacitor 2 is charged by the output of the operational amplifier 6 via the second resistor 8 and the first resistor 4, so that the inverted input voltage of the operational amplifier 6 equals the reference voltage V.sub.REF (see y in FIG. 2(3)). As a result, the time required for amplifying signals based on the reference voltage can be somewhat shortened by connecting the diode 102.
The conventional amplifier described above does have disadvantages. First, a long time elapses before the second capacitor 14 charges the reference voltage, because it is performed via the third resistor 10. Second, the first capacitor 2 is shut off before it obtains the reference voltage V.sub.REF due to the forward voltage V.sub.F of the diode 102. As a result, there is a considerable time lag before the noninverted input voltage of the operational amplifier 6 equals reference voltage V.sub.REF after second capacitor 14 fully charges. Consequently, a long time is required before signals are amplified.
The ways to shorten the time required for amplifying signals are as follows. The first way is to lower the second resistor 8 to smaller level. The second way is to lower the third resistor 10 and the fourth resistor 12 to smaller level. The third way is to downgrade the capacity of the second capacitor 14.
Unfortunately, the first way reduces the amplification factor A. The second way increases power consumption in the third resistor 10 and the fourth resistor 12. The third way makes the reference voltage V.sub.REF less stable. For these reasons, it is impossible to take these steps.